With the popularization of wireless communication terminals such as smartphones, traffics between a wireless base station and a housing stations increase. Therefore, the application of the transmission technology of DWDM (Dense Wavelength Divisional Multiplexing) to a metro-access system network is considered. In such a network, a transmission device of a central side (backbone side) and a plurality of transmission devices of an end-user side are connected to each other via an AWG (Arrayed Waveguide Grating) that demultiplexes optical signals with multiple wavelengths, and configures a star type network such as a PON (Passive Optical Network). In the following, the transmission device of the central side is referred to as a “central side device”, and the transmission device of the end-user side is referred to as a “user side device”.
The user side device can transmits the optical signal with a different wavelength for each user side device, and a wavelength variable light source is provided in each user side device so that a versatility between the respective user side devices is ensured. The central side device superimposes control data for a wavelength setting on a main signal to be transmitted to each subscriber device in order to set a wavelength of the wavelength variable light source for each user side device. Regarding the superposition of the signal, for example, Patent Document 1 (i.e., Japanese Laid-open Patent Publication No. 5-244046) discloses superimposing monitoring data on the optical signal.